WO2003046227A1

WO2003046227A1 - Method for separating xylose from lignocelluloses rich in xylan, in particular wood

Info

Publication number: WO2003046227A1
Application number: PCT/EP2002/013377
Authority: WO
Inventors: Ties Karstens
Original assignee: Rhodia Acetow Gmbh; Ties Karstens
Priority date: 2001-11-27
Filing date: 2002-11-27
Publication date: 2003-06-05
Also published as: US20050065336A1; DE10158120A1; AU2002358051A1; EP1448801A1

Abstract

The invention relates to a method for separating xylose from lignocelluloses rich in xylan, in particular wood and for obtaining cellulose. Said method is characterised by the steps: (1) pretreatment of chips by the mechanical destruction of their original structure, (2) impregnation of the obtained wood weight with a diluted mineral acid, (3) prehydrolysis of the modified wood weight that has been obtained by the action of water vapour at a raised temperature, in order to hydrolyse the hemicelluloses obtained, and (4) removal of the hemicelluloses from the residual cellulose by washing, filtration and/or centrifugation to obtain a solution rich in xylose. The combination of inventive steps allows high α-cellulose concentrations to be obtained with extremely low xylose fractions, i.e. highly pure chemical cellulose qualities, whilst at the same time a near quantitative separation of the valuable xylose can be achieved.

Description

Process for separating xylose from xylan-rich lignocelluloses, especially wood

The invention relates to a process for separating xylose from xylan-rich lignocelluloses, in particular wood, and for obtaining pulp.

For the industrial utilization of wood, this natural composite of the framework substance in the form of cellulose chains twisted to long and solid fibrils and the hydrophobic lignin acting as a protective covering must be destroyed in order to then separate these components from one another in what is known as wood digestion. Another essential part of the wood are the hemicelluloses (polyoses). The aim is mostly to obtain more or less pure cellulose, with the other constituents lignin and hemicelluloses, mostly in degraded form, only as

By-products are incurred.

In the case of cellulose, a distinction is made between paper cellulose, which in addition to the main constituent α-cellulose can contain up to 25% hemicellulose, and higher quality

^" V. Chemical pulp with an α-cellulose content of over 90%, the proportion of hemicellulose should not exceed a few percent. As is known, chemical pulp is one of the most important cost factors in the production of cellulose acetate, filter tow and other high-quality cellulose products.

To produce such chemical pulps from wood or comparable biomass, besides removing the lignin, it is also necessary to remove the hemicelluloses as far as possible. The hemicelluloses are becoming increasingly important due to their possible uses.

Numerous processes for the chemical degradation and removal of xylose from wood have been developed in the prior art. A classic method for removing xylose uses acidic pre-hydrolysis, which in practice is carried out either with dilute sulfuric acid in a concentration of 0.1 to 0.5% and a temperature of 90 to 140 ° C or in pure water at a temperature from 165 to 175 ° C is carried out (H. Sixta, G. Schild and Th. Baidinger: Das Papier Heft 9, 1992, p. 527 ff). About 70% of the xylose is considered typical for the removal during the pre-hydrolysis. Of the remaining 30%, 40 to 50%, ie absolutely 12 to 15%, of the xylose are removed during the cooking. Accordingly, a total of 82 to 85% of the xylose present is removed from the wood.

The hydrolysis is called water hydrolysis, i.e. carried out at liquor ratios of 3 to 4: 1. The heating-up time is at least 60 minutes, usually up to 90 minutes. The hydrolyzate is usually disposed of thermally together with the cooking liquor. In order to reduce the high energy costs associated with the evaporation of the pre-hydrolysates, there has been no lack of attempts to reduce the liquor ratio down to pure steam pre-hydrolysis (liquor ratio 1: 1 to 1: 5.1). Unfortunately, this technologically very simple process has a very negative impact on the pulp quality. Studies have shown that steam pre-hydrolysis is the clear cause of higher kappa numbers, poorer bleachability, lower alkali resistance and reactivity of pulps.

Prehydrolysis is also limited by the occurrence of side reactions that are difficult to control. The most important side reaction, the dehydration of the pentoses to furfural, is the starting point for the undesired inter- and intramolecular condensation reactions. Resin-like compounds are formed which, as the reaction continues, separate from the aqueous phase and can be deposited on all existing surfaces (obstruction and blockages during operation).

According to the article by H. Sixta, G. Schild and Th. Baidinger mentioned above and the pre-hydrolysis described there at temperatures of 164, 170, 178 and 185 ° C., the maximum achievable xylose concentrations after 100 minutes are summarized in Table 1 below. At all reaction temperatures - with the exception of 164 ° C - more furfural than xylose is formed. The furfural concentration reaches a saturation value of 20 to 23 g / 1. Reaction conditions in which the undesired further reactions of the furfural to condensation products can still be controlled to an extent that is acceptable for technical processes are at the maximum pre-hydrolysis times shown in Table 2. Table 1 Table 2

Hydrolysis hydrolysis time to reach hydrolysis maximum temperature of the highest xylose temperature controllable concentration of 1 14-I5g / 1 hydrolysis time

[° C] [min] [° C] [min]

164 180 165 160

170 140 171 90

178 70 179 60

185 20 186 30

The P factor for these temperatures and times is approx. 700 + 5. The P-factor model is used to calculate the degree of pre-hydrolysis using the time-temperature profile. Temperature and time are combined in one variable, the P-factor. In this respect, the P-factor can be seen analogously to the severity factor introduced in the steam explosion of wood. According to the above article by H. Sixta, G. Schild and Th. Baidinger, the viscosity decreases with this P-factor by approx. 20%. If the P factor is observed, i.e. In the above temperatures and times, the yield of undesirable by-products is low. The glucose concentration is largely independent of the maximum reaction temperature, and is approximately 2 g / 1.

In addition, the known sources for xylose show a number of disadvantages. Known sources of xylose are, for example, the cooking liquids of the Mg and Ca sulfite processes. However, these must be subjected to an ion exchange in order to remove the lignin sulfonates, further chromatographic methods for separating the xylose from other sugar components, possibly crystallizations, being necessary.

It is known that hardwood paper pulps are subjected to an alkali treatment (8-12% NaOH) at 60 to 80 ° C. in order to convert the xylan into the liquid phase. This xylan must then be subjected to hydrolysis to xylose. A slurry after treatment with xylanase also results in xylan hydrolyzate, from which xylose must be enriched by chromatographic separation methods. In addition, the sulfite process ("cooking liquor" or "spent liquor") initially leads to a separation by Ca salt chromatography (ion exchanger) into a first lignosulfonate fraction and into a sugar-rich fraction. This is followed by a second separation by Na salt chromatography (ion exchanger) into a further lignin sulfonate fraction and into a purified sugar-rich fraction.

Accordingly, there is a need for a simple, economical process to separate the hemicelluloses, especially xylose, from wood while maintaining the cellulose.

Against the background of the prior art described above, the object of the invention was therefore to provide a process for separating xylose which works in a simple and economical manner, with as little waste products as possible being processed. Furthermore, a high cellulose yield should be achieved with the method according to the invention.

This object is achieved according to the invention by a method for separating xylose from xylan-rich lignocelluloses, in particular wood, and for obtaining pulp, characterized by the steps: (1) pretreating wood chips by mechanically destroying the original structure,

(2) treating the wood mass obtained with dilute mineral acid,

(3) performing a pre-hydrolysis of the process-modified wood mass obtained under the action of steam at elevated temperature in order to hydrolyze the hemicelluloses contained, and

(4) Remove the hemicelluloses from the remaining pulp by washing, filtering and / or centrifuging to obtain an xylose-rich aqueous solution.

The invention allows the above-described object to be achieved in an advantageous manner. It therefore contributes to a considerable improvement in the state of the art. A completely different procedure than in the conventional wood pulping process succeeds in achieving numerous advantages over sulfite or prehydrolysis sulfate processing. drive to lead into the field. This is primarily due to significant improvements in the treatment of wood chips. These are based on the avoidance of the disadvantages that arise with classic cooker technology, namely infestation of cookers with the chips, long heating and cooking times, high liquor ratios and hardly solvable problems with regard to heat and material exchange due to low diffusion speeds and negligible thermodynamic potential differences. These in turn necessitate high investment requirements, ie large parts of the plant due to long dwell times.

According to the invention, the prehydrolysis is therefore not carried out on the chips themselves, but after their pretreatment. This pretreatment consists in first squeezing and shearing the wood chips in process step (1) by means of a suitable mechanical process engineering unit, such as a press screw, screw press or the like, so that their original structure is destroyed. This massive mechanical stress not only removes water stored in the wood chips. The air is also removed from the capillaries of the wood. In addition, the moisture is significantly reduced by the squeezing process, namely by up to about 50%.

Any comminuted plant growth materials of various types can be used, such as wood, oat husks, maize or grain stalks, bagasse, wheat straw, rice, straw and oat straw. In the case of wood, it is common to use logs or small pieces of wood in shredded form. For fibrous raw materials, such as annual plants, cut fibers are suitable. Shredded, in particular shredded, wood is preferably used, preference being given to the use of hardwood, in particular beech wood or eucalyptus.

The mechanical stress after step (1) of the method according to the invention enables the wood mass obtained; wood chips can no longer be spoken after this intervention, soaking up spontaneously and in a very short time with any type of liquid offered, in order to approximate the original state again , This fact can now be exploited by impregnating the wood chips pretreated in this way in accordance with step (2) of the process of the invention with dilute mineral acid. The mineral acid can be any known mineral acid. Examples include: sulfuric acid, hydrochloric acid and / or nitric acid, with sulfuric acid being particularly preferred. Advantageously, about 0.05 N to

2.5 N, in particular about 0.1 N to 1.5 N, dilute aqueous mineral acid solution used.

This is soaked up spontaneously by the wood mass to such an extent that its liquid content after being vacuumed up is preferably between about 55 and 70%.

The uniformity of the distribution over the entire cross-section of the wood mass can be easily recognized by the discoloration of the wood mass from light to dark. The distribution of the liquid is usually even, unless the wood chips have not been squeezed sufficiently.

According to step (3), the wood mass pretreated in this way can be treated in a suitable apparatus, such as, for example, an "inclined screw reactor", Pandia reactor or the like, by treating with steam at a temperature of about 140 to 180 ° C., in particular about 150 to Treat for about 5 to 40 minutes, in particular about 5 to 20 minutes, preferably in a continuous mode of operation, with prehydrolyzate also being able to be taken off continuously.

The process-modified wood mass obtained after carrying out the pre-hydrolysis according to step (3) can expediently be obtained in a very simple manner and without a large amount

Effort to be fully unraveled. This is achieved, for example, by means of a refiner, a toothed disk mill, strong shear in a narrow tube as a result of the reduction in steam pressure from the temperatures mentioned to normal pressure and the like. The separation of pulp and steam is preferably carried out in a cyclone.

The hemicelluloses still adhering to the fibers and broken down into sugars can then be separated quantitatively in step (4) in a washing or filtration step. to Laundry is preferably a washing centrifuge, a double washing press, a belt press or, in general, a multi-stage washing unit.

The process is particularly effective for woods with a high xylan content, e.g. Eucalyptus pulp. Accordingly, about 10 to 25%, in particular about 15 to 20%, aqueous xylose solutions can advantageously be obtained. In these, a super-saturated solution can be obtained by further concentration, from which the xylose can be crystallized. Experience has shown, however, that 25% xylose solutions are economically interesting for further processing.

Accordingly, the combination of the process steps according to the invention makes it possible to achieve high α-cellulose contents with very low xylose fractions, that is to say high-purity chemical pulp grades, and at the same time an almost quantitative separation of the valuable xylose is possible.

Further advantages of the process according to the invention are a shorter Auflieiz- and reaction time in the pre-hydrolysis and a low liquor ratio of water / chips in the first stage of the process. Water vapor is also used, which has the advantage that the degraded hemicelluloses are more concentrated

Form as if you were to perform a water pre-hydrolysis. Because essentially the steam condensate falls off as the pre-hydrolyzate.

A pre-hydrolysis of the wood chips according to the method according to the invention also means a considerable saving of time and costs compared to conventional methods. In addition, the hydrolyzate contains the sugar in a high concentration of about 12 to 14%, which is also a favorable basis for further processing or, if desired, a concentration. After a countercurrent wash, for example, the wood pulps only contain about 10% or less of the original hemicelluloses - depending on the intensity of the impregnation and the pre-hydrolysis conditions. Accordingly, the hemiceluloses can be almost completely removed from the wood, with no undesirable mixing of lignin and carbohydrates. Rather, a clear separation of the essential wood components in pure form is obtained from one another, ie an effective separation of all soluble carbohydrates is achieved in one separation step, with low cellulose losses also occurring.

Continuous operation is also possible according to the invention. Disposal problems with regard to wastewater disposal, incineration and the like are eliminated. In addition, additional process steps for processing the hemicelluloses for their disposal can be omitted. Additional costs for the disposal of waste products can also be avoided.

In addition, valuable products in the form of hemicelluloses, in particular xylan, are obtained, which, if desired, can be refined accordingly. A

Marking the xylose for further processing to xylitol (xylitol) is also of great importance. This contributes significantly to improving the economy of the method of the invention. According to the invention, a prehydrolysate free of foreign substances is obtained, high xylose concentrations being achieved in the prehydrolysate. Complex xylose enrichment is generally not necessary and the xylose obtained can also be decolorized. A separation of lignosulfonates that is regularly necessary in the prior art can also be omitted.

Furthermore, furfural formation is largely avoided by the method according to the invention, so that there is no loss of yield in this regard. Instead, the desired high yields of hemicelluloses broken down into sugars and of α-cellulose are obtained.

The invention is illustrated by the following examples. Examples

Some embodiments of the method according to the invention are to be illustrated in detail with the aid of the attached FIGS. 1 and 2.

As shown in Figure 1, evenly shredded and dirt-free industrial wood chips (1) are first moved over the MSD (2), whereby both the wood structure is largely destroyed and the natural moisture considerably (from approx. 50%> to approx. 20-25 % o) is reduced. MSD is a screw press with the characteristic that the shaft of the screw compresses with increasing distance from the feed point of the wood chips. This means that the wood chips have less free space. The worm rotates slowly, with the wood chips being conveyed in a perforated steel sheet. The water escaping from squeezing / pressing the wood chips can be drained through the perforated steel sheet.

As a result of the treatment set out above, the wood chips are receptive to the dilute acid (3), which, indicated by the arrow, is added and is required in the pre-hydrolysis to break down the hemicelluloses contained in the wood. In the present case, a beech wood sample was impregnated with a 0.6% sulfuric acid and then hydrolyzed at 160 ° C. for 20 minutes.

Using a perforated conveyor belt or a perforated trough with an Archimedean screw, the wood chips impregnated with acid can be conveyed to the feed hopper of the digester (4).

There they are continuously introduced into the pressure range of the digester (5), which is under steam pressure, via a feed screw and a ball valve lock. Due to the obviously very uniform distribution of the acid, the hydrolysis of the

Wood chips can be done completely in just twenty minutes with steam. The wood chips are conveyed through the transport screw built into the digester and are continuously discharged via a blow valve connected to one Cyclone emptied. Alternatively, a refiner or a decnotter can also be provided in order to fiber up well-digested parts but still have the wood chip structure. A decnotter is a machine for shredding non-fibrillated, steam-treated wood chips. The hydrolyzate containing the degraded hemicelluloses in the form of monosugars must also be continuously withdrawn at the lower end of the digest. The possibility of continuously monitoring the hydrolyzate composition also lends itself to a production plant. This measurement signal can be used for statistical process control.

Good fibering, for example in a refiner (6), of the prehydrolysed wood chips is advantageous for the subsequent washing step (7), in which the sugars still adhering to the fibers are separated off quantitatively. A multi-stage countercurrent wash is advantageous here. A separation can also be carried out by centrifugation, which can be sufficient for the quantitative removal of the soluble sugars from the prehydrolyzed wood, possibly in combination with one or two washing stages in the centrifuge.

The influence of the sulfuric acid concentration on the xylose content in a beechwood is plotted in FIG. In particular, FIG. 2 shows that the pre-hydrolysis step with MSD / impregnation and digester explained in FIG. 1 is reproducible on the one hand and that there is also a linear relationship between the sulfuric acid concentration in the impregnation bath and the xylose content in the fiber after washing, which can be used for a corresponding optimization ,

An analysis of the cellulose yield in a beech wood treated with the method according to the invention and subsequent 5-stage washing led to the following results: Table 3:

The cellulose losses - measured as glucose - are probably even lower, because glucomannan can also be used as a glucose source in the pre-hydrolysis, which is certainly more easily broken down than the cellulose chains themselves. Accordingly, the yield is much higher than in the case of processes from the prior art which the basic component is sodium hydroxide solution.

The xylose content of 12.3 kg of beech wood used was also determined. The complete composition of the beech wood results from the additional information in "Das Papier" 53 (2), (1999) 101:

Table 4:

The results obtained are summarized in Table 5 below: Table 5:

*) 12.3 kg beech wood x 0.28; including glucuronic acid

One can therefore speak of a satisfactory agreement (approx. 3.5% deviation) when comparing the calculated xylose content of the beech on the one hand and the sum of the xylose contents in the fibrous material and in condensate and wash water. When using a 0.6% sulfuric acid in the impregnation, there is still a residual xylose content of about 5% after washing. This allowed 78% of the xylose contained in beech wood to be grown. The proportion of xylose in the dissolved carbohydrates is between 68 and 75% depending on the test conditions.

This result represents a significant improvement over the prior art.

H Φ

Claims

claims

1. A method for separating xylose from xylan-rich lignocelluloses, in particular wood, and for obtaining pulp, characterized by the steps:

(1) pretreating wood chips by mechanically destroying the original structure,

(2) impregnating the wood mass obtained with dilute mineral acid, (3) performing a pre-hydrolysis of the process-modified obtained

Wood mass under the action of steam at elevated temperature to hydrolyze the hemicelluloses contained, and (4) removing the hemicelluloses from the remaining pulp by washing, filtering and / or centrifuging to obtain a xylose-rich aqueous solution.

2. The method according to claim 1, characterized in that hardwood, in particular beech wood or eucalyptus, is used as wood for the wood chips.

3. The method according to claim 1 or 2, characterized in that the mechanical destruction after step (1) is carried out with crushing and / or shearing the chips.

4. The method according to claim 3, characterized in that a mechanical screw or a screw press is used for mechanical destruction after step (1).

5. The method according to at least one of claims 1 to 4, characterized in that sulfuric acid, hydrochloric acid and / or nitric acid is used as the mineral acid in step (2).

6. Verfaliren according to claim 5, characterized in that an approximately 0.05 N to 2.5 N, in particular approximately 0.1 N to 1.5 N diluted aqueous mineral acid solution is used.

7. The method according to at least one of claims 1 to 6, characterized in that the treatment with steam in step (3) at a temperature of about 140 to 180 ° C, in particular about 150 to 180 ° C, for about 5 to 40 minutes , in particular about 5 to 20 minutes.

8. The method according to at least one of the preceding claims, characterized in that the process-modified wood mass is further fiberized after step (3).

9. Verfaliren according to claim 8, characterized in that the unraveling is carried out with a refiner, a toothed disk mill or via a steam blow line.

10. The method according to at least one of claims 1 to 9, characterized in that in step (4) is washed with hot water or ammonia water.

11. The method according to claim 10, characterized in that the laundry is carried out as a single or multi-stage countercurrent wash.

* * *